Automakers used to make their rides quieter with padding, special bushings, and thicker glass. But now that fuel-efficiency standards are forcing them to jettison every unneeded gram, they are leaning toward the active alternative: electronic antinoise.
Bose pioneered antinoise, first in audio headphones and more recently in cars, where its system cancels low-frequency noise from the engine by using the speakers to broadcast a wavepattern half a cycle out of phase with that noise. When the peaks of one wave meet the troughs of the other, they cancel out. The rest is silence.
Now comes Harman, of Stamford, Conn., with a system that tackles road noise. The problem was that this kind of sound doesn’t come from a regularly reciprocating engine and so can’t be predicted in a straightforward way. But Harman’s engineers found a trick.
“We have accelerometers mounted on points in the car, selected to have a high correlation with noise,” explains David Trumpy, an audio engineer at Harman’s research center in Farmington, Mich. “The reaction of the air space in the cabin of car can be predicted based on the analysis we do on a vehicle’s structure.”
Events, like hitting a bump in the road, must be registered and acted on in milliseconds, Trumpy says. But, as with engine noise, the frequency is low and the waves are long, “so you can create a decent-size zone of cancellation in the cabin.” Wind noise creates waves so short that a movement of your head can muss up the antinoise effect.
Harman’s research partner, Lotus Engineering, of Britain, said in a press release that automotive antinoise could be considered a spinoff of its own work in the 1980s on active suspensions for Formula 1 cars. Active suspensions model the basic cause of road noise—the varying connection between wheels and pavement.
Both Harman and Lotus Engineering declined to say when the road-quietening system would debut, or in which car.
Bose wouldn’t say whether it planned a road-noise antidote of its own. However, a spokesman did point point out that the company’s engine-noise canceller is already available in many cars, including models from Cadillac, Buick, Chevrolet, Infiniti and Nissan.